Fatigue Fracture Behavior of Bearing Steel GCr15 in Very High Cycle Regime

Abstract:

Article Preview

In order to investigate the fatigue behaviors in very high cycle fatigue regime of bearing
steel GCr15 (Chinese standard), cantilever-type rotary bending fatigue test was carried out, by using
an hourglass-shaped specimen. As a result, the S-N data were classified into surface grinding flaw
or inclusion induced fracture mode, subsurface inclusion without ‘granular-bright-facet’ (GBF) area
induced fracture mode and subsurface inclusion with GBF area induced fracture mode. Formation
of a GBF area was clearly observed in the vicinity of subsurface inclusion on the fracture surface
ruptured at high- cycle fatigue regime. The cause of the S-N curves of GCr15 obtained by this
experiment continuously decreased, which is different from the common ‘duplex S-N’ curve.
Experiments on fracture surface was carried out based on the ‘dispersive decohesion of spherical
carbide’ model aimed to clarify the mechanism of the formation of GBF area.

Abstract: A roller mill is composed of several rollers, rotational table liners, hydraulic cylinders and
raw materials are ground between the rollers and the table with the assistance with a shearing and
compressive force. It has been reported that an unexpected fatigue failure occurred in a table liner in
the course of grinding portland cement. It is subjected to the cyclic bending stress by the rollers load
and the centrifugal force by the table rotation and fractures at the edge of grinding path of outer roller.
It demands design life 4×107 cycles but has fatigue life 4×106 ~ 8×106 cycles. The purpose of this
study is to reveal the failure mechanism of the table liner occurring in the grinding operation of roller
mill and to show the structural design guidelines for reducing the fatigue failure.

Abstract: Gigacycle fatigue behavior in high-strength steels tested under rotary bending fatigue
was summarized in this paper. Characteristic of the very high cycle fatigue is to be caused the
transition of fracture mode from surface-induced fracture to subsurface inclusion-induced one. In
the vicinity of an inclusion at the origin of internal crack, granular-bright-facet (GBF) area was
formed during extremely long fatigue cycles. It was pointed out that the formation of GBF area was
an important factor for the control of the internal fatigue fracture in gigacycle regime. The GBF
area revealed a very rough granular morphology compared with the area outside the GBF inside the
fish-eye zone, and was related to the carbide distribution in the microstructure of the matrix. From
the detailed observation of fracture surface and computer simulation by FRASTA method, the GBF
area formation mechanism in a gigacycle fatigue regime was proposed as the ‘dispersive
decohesion of spherical carbide model’.

Abstract: In this research, fatigue behaviour of Ti-6Al-4V alloy was investigated for smooth and notched specimens with stress concentration factor(Kt) 3.6 and 4.1.This investigation was conducted for various diameter bars having different ultimate strength.Rotating bending fatigue test at R= -1 was emploied for this research. Notch sensitivity data was compared with those of steels. The result indicated that the presence of notch in this alloy has a different amount of sensitivity when the notch specimens were subjected to high cycle fatigue (HCF) and low cycle fatigue(LCF) tests.The notch sensitivity of this alloy was shown generally to be much lower than steel alloys with similar ultimate strength values. Therefore,considering the low sensitivity to notch of this alloy, can be recommended for applications with the presence of notch such as biomedical application

Abstract: Using the ultrasonic fatigue test method, the influence of subjection to plasma nitriding surface modifications on the ultra-high cycle fatigue behavior of Ti-6Al-4V was investigated, then a comparison with corresponding behaviors of the normal Ti-6Al-4V was made. The results show the S-N curve of Ti-6Al-4V with plasma nitriding surface modifications still continuously declines, no conventional fatigue limit exists for Ti-6Al-4V. Plasma nitriding surface modifications enhance surface hardness, but they make the material fatigue strength decrease by about 17 %. The fractography of fatigue failure has been observed by scanning electron microscopy. The observation shows that fatigue failure initiates from internal of specimen after the fatigue life of 108 cycles and fatigue cracks mainly initiate from the surface of specimen before the fatigue life of 108 cycles.

Abstract: Titanium (Ti) alloys are used in critical, fatigue limited applications in aircraft and aircraft engines. Current design practices are, of necessity, conservative in order to minimize risk of unexpected failures. Among the sources of this conservatism are the inherent variations in the number of loading cycles the materials can withstand prior to fatigue crack initiation, the uncertainty in crack propagation lifetime prediction and the need to set safe minimum component life values. While the stochastic nature of fatigue is well-known, improved characterization methods have begun to provide a better understanding of the crack initiation process. This paper describes recent work designed to provide an improved understanding of the relationship between thermomechanical processing history, microstructure, texture and the fatigue behavior of α+β Ti alloys. Due to length limitations, the paper focuses on two important aspects of fatigue life variation: the effects of microstructural discontinuities on fatigue and the role of facet formation during crack initiation and the early stages of fatigue crack growth.